FIGS. 1 and 2 show the structure of a counter-rotating axial flow fan disclosed in Japanese Patent No. 4128194. FIGS. 1A, 1B, 1C, and 1D are respectively a perspective view as viewed from a suction side, a perspective view as viewed from a discharge side, a front view as viewed from the suction side, and a rear view as viewed from the discharge side, of the counter-rotating axial flow fan according to the related art. FIG. 2 is a vertical cross-sectional view of the counter-rotating axial flow fan of FIG. 1. The counter-rotating axial flow fan is constructed by assembling a first axial flow fan unit 1 and a second axial flow fan unit 3 via a coupling structure. The first axial flow fan unit 1 includes a first casing 5, and a first impeller (front impeller) 7, a first motor 25, and three webs 21 disposed in the first casing 5. The webs 21 are arranged at intervals of 120° in the circumferential direction. The first casing 5 has an annular flange 9 on the suction side in the direction in which the axial line A extends (in the axial direction), and an annular flange 11 on the discharge side, which is opposite to the suction side, in the axial direction. The first casing 5 also has a cylindrical portion 13 between the flanges 9 and 11. The internal spaces in the flange 9, the flange 11, and the cylindrical portion 13 form an air channel. The flange 11 on the discharge side has a circular opening portion 17 formed therein. The three webs 21 of the first axial flow fan unit 1 are assembled with three webs 45 of the second axial flow fan unit 3 to form three stationary blades 61 as explained later. The first motor 25 rotates the first impeller 7 in the first casing 5 in the counterclockwise direction in FIG. 1C (in the direction of the arrow R1 on the paper, which will be referred to as “one direction R1”). The first motor 25 rotates the first impeller 7 at a rotational speed higher than the rotational speed of a second impeller (rear impeller) 35 as explained later. The first impeller 7 has an annular member (hub) 27 fitted with a cup-shaped member of a rotor (not shown) fixed to a rotary shaft (not shown) of the first motor 25, and N (five) front blades 28 integrally provided on an outer peripheral surface of an annular peripheral wall 27a of the annular member 27.
The second axial flow fan unit 3 includes a second casing 33, and a second impeller (rear impeller) 35, a second motor 49, and three webs 45 disposed in the second casing 33 and shown in FIG. 2. As shown in FIG. 1, the second casing 33 has a flange 37 on the suction side in the direction in which the axial line A extends (in the axial direction), and a flange 39 on the discharge side, which is opposite to the suction side, in the axial direction. The second casing 33 also has a cylindrical portion 41 between the flanges 37 and 39. The internal spaces in the flange 37, the flange 39, and the cylindrical portion 41 form an air channel. The first casing 5 and the second casing 33 form a case. The flange 37 on the suction side has a circular opening portion 42 formed therein. The second motor 49 rotates the second impeller 35 in the second casing 33 in the counterclockwise direction in FIGS. 1B and 1D or in the direction of the arrow R2 on the paper, which will be referred to as “other direction R2”, that is, in the direction opposite to the direction of rotation of the first impeller 7 (the direction of the arrow R1). As explained earlier, the second impeller 35 is rotated at a rotational speed lower than the rotational speed of the first impeller 7. The second impeller 35 has an annular member (hub) 50 fitted with a cup-shaped member of a rotor (not shown, fixed to a rotary shaft (not shown) of the second motor 49, and P (four) rear blades 51 integrally provided on an outer peripheral surface of an annular peripheral wall 50a of the annular member 50.
The front blades 28 each have a curved shape in which a concave portion opens toward the one direction R1 as viewed in lateral cross section. The rear blades 51 each have a curved shape in which a concave portion opens toward the other direction R2 as viewed in lateral cross section. The stationary blades (support members) 61 each have a curved shape in which a concave portion opens toward the other direction R2 and toward the direction in which the rear blades 51 are located as viewed in lateral cross section.
In the counter-rotating axial flow fan, the number N of the front blades 28, the number M of the stationary blades 61, and the number P of the rear blades 51 are each a positive integer, and satisfy a relationship of N>P>M. In the counter-rotating axial flow fan, as shown in FIG. 2, the length (maximum axial chord length) L1 of the N front blades 28 of the first axial flow fan unit 1 as measured along the direction of the axial line A is set to be larger than the length (maximum axial chord length) L2 of the P rear blades 51 of the second axial flow fan unit 3 as measured along the direction of the axial line A. Specifically, the two lengths L1 and L2 are determined such that the ratio L1/L2 of the length L1 to the length L2 is a value of 1.3 to 2.5 to improve the air flow—static pressure characteristics.
While the conventional counter-rotating axial flow fan can improve the air flow—static pressure characteristics, it is desired to further improve the characteristics and reduce noise.